Reprocessing of a used nuclear fuel is made to recover and purify uranium and plutonium that can be used once again as a nuclear fuel and to remove and discard other radioactive substances. It is a wet process referred to as a "Purex process" that has been most commonly employed as the reprocessing method at present. This method comprises primarily a dissolving step of dissolving the used nuclear fuel in nitric acid, a co-decontamination step of separating uranium and plutonium from other radioactive substances by solvent extraction from this nitric acid solution, a distribution step of separating uranium and plutonium thus extracted, and a purification step of purifying uranium and plutonium, respectively. Besides these main steps, there are a step of recovering nitric acid that can be used for the main steps from the nitric acid solution containing nuclear fission products from the each step and a step of processing them as wastes. Therefore, almost all of the steps of the reprocessing process are exposed to the nitric acid solution. For this reason, stainless steel having a reduced carbon content (e.g. SUS304L, SUS316L or SUS310 system stainless steel), which is believed to be highly resistant to corrosion by nitric acid, has been employed in the past as countermeasure for corrosion from the material aspect. However, if any nuclear fission products are contained in this nitric acid solution, corrosiveness of the nitric acid solution is enhanced remarkably and the problem of corrosion occurs even for the stainless steel described above which exhibits sufficiently high corrosion resistance to nitric acid alone.
Generally, stainless steel has a protective oxide film on the surface in a system consisting of nitric acid alone and is under the passive state but if any chemical seed having high oxidation force such as Cr (VI) ion is contained therein, the oxidation force of the nitric acid solution is remarkably enhanced Accordingly, when the surface potential of the stainless steel becomes remarkably high and reaches a certain specific potential beyond the passive state potential, the metal dissolves as a high valency ion and there occurs a phenomenon that promotes vigorously corrosion, that is, so-called "over-passive state corrosion". In the nuclear fuel reprocessing plant, the nuclear fission products might exist in the nitric acid solution at all of these steps, though the concentration might change, and the Ru ion of the platinum group and the Ce ion of the rare earth element that can operate as the chemical seed having high oxidation force are contained. Furthermore, Cr and Fe ions as the corrosion products from the stainless steel itself might exist in nitric acid. Among others, these ions are likely to assume a high oxidation state at positions where the nitric acid concentration is high or the temperature of the nitric acid solution is high and they can exist as Ru(VIII), Ce(IV), Cr(VI) and Fe(III). Accordingly, it can be said that ordinary stainless steel is highly likely to cause the over-passive state corrosion. Among the problems of corrosion of stainless steel in the reprocessing plant, therefore, corrosion resulting from the co-existing chemical seeds having high oxidizability is believed important. In such a high oxidizing atmosphere, a material of a 17%Cr - 14%Ni - 4%Si system and a material of an 8%Cr - 20%Ni - 6%Si system, that have an increased Si content, have been proposed Journal of Japan Metallurgical Society, Vol 22, No. 4, pp. 320-322, 1983).
Japanese Patent Laid-Open No. 46380/1985 discloses a method which blows a NOx gas into a nitric acid solution as a method of preventing corrosion by the nitric acid solution in a nitric acid production plant, a nitric acid oxidation apparatus, a reprocessing plant of radioisotopes, and the like.
In the high Si system stainless steel such as described above, intergranular corrosion which is the characterizing feature of the over-passive state corrosion of ordinary stainless steel such as SUS304L system and the SUS310 system does not occur but the whole-surface corrosion occurs. However, corrosion resistance of such a steel is not always sufficient in a solution consisting of a medium or high concentration nitric acid alone, and excellent machinability and weldability of stainless steel will be lost if a large amount of Si is added. Accordingly, it has been difficult to obtain sufficient corrosion resistance without losing machinability in a highly oxidizing nitric acid environment by the improvement from the material aspect alone.
The method of blowing the NOx gas cannot easily control the reducing reaction and involves the possibility that the metal ions as the radioactive nuclei are precipitated as the metals. If such ions are precipitated as the metals or if a precious metal such as platinum is precipitated, corrosion of the materials of the apparatuses might be accelerated remarkably by galvanic corrosion so that processing apparatuses for such ions are necessary.
Incidentally, U.S. Pat. No. 3,891,741 proposes a method which causes the precipitation on the cathode surface by electrolytic reduction to recover nuclear fission products such as Ru but since this method is the method of precipitating the metals, it is not free from possible galvanic corrosion.